High-Frequency Ventilation- Basics and Practical Applications

Indications; HFV+IMV
5 Combining HFV and IMV, and
‘sustained inflation’
Oscillatory ventilation on its own can be used in the CPAP mode, or
with superimposed IMV strokes, usually at a rate of 3 to 5 strokes per
minute (cf. appendix 12.1). The benefit of the IMV breaths is probably
due to the opening of uninflated lung units to achieve further ‘volume
recruitment’.
Sometimes very long inspiratory times (15 to 30 s) are suggested for
these sustained inflations (SI). By applying them about every 20 minutes
compliance and oxygenation have been improved and atelectases
prevented (cf. figure 5.1), [10, 11, 37, 41, 65, 70, 113]. Especially after
volume loss by deflation during suctioning the lung soon can be reopened
with a sustained inflation. How ever, whether these inflation
manoeuvres should be employed routinely is subject of controversial
discussions. In most of the clinical studies no sustained inflations
were applied. In animal trials no increased incidence of barotrauma
was found [10].
Prevention of atelectases, which might occur under HFV with insufficient
MAP (cf. 9.1 complications), is the primary benefit of combining
HFV and IMV. HFV superimposed to a normal IMV can markedly
improve CO 2 washout (‘flushing the deadspace’ by HFV) at lower peak
pressures than in conventional ventilation [7, 8, 9, 12, 44, 50, 109].
22